This invention relates to passive generation of magnetic bubbles in magnetic bubble devices.
Magnetic bubbles are mobile, single wall, cylindrical domains of magnetization found in specially formed thin planar slabs of magnetic materials when these materials are subjected to a bias field perpendicular to the plane of the material. The bubbles or cylindrical single wall domains are magnetized in a direction opposite to the bias field magnetization of the material and parallel to the direction of the bias field. Magnetic bubble devices are used as computer memories, the presence or absence of a bubble at a particular location within the material representing a one or a zero. This memory technology promises to provide high density memories (greater than 10.sup.6 bits per in.sup.2) with fast access times.
In the demagnetized state, i.e, in the absence of a bias field, the thin layer of magnetic material possesses a plurality of strip wall domains, i.e. a plurality of meandering quasi-parallel regions of uniform magnetization. Each domain is inherently magnetized in a direction normal to the plane of the material with approximately one half of the domains being magnetized oppositely to the other half. As the bias field strength is increased a strip-bubble transition field value or strip-out field value is reached where strip single wall domains contract into cylindrical single wall or bubble domains, i.e., the strip single wall domains close on themselves and form cylinders of reverse magnetization in the material having roughly circular cross sections. As the bias field strength is increased further the diameters of the bubble domains decrease until a saturation field value is reached at which value all bubble domains collapse and the layer of material becomes saturated into one domain of magnetization.
In the prior art, once bubbles are generated they can be maintained in a stable condition by application of the bias field at operating strengths between the strip-out and saturation field values. At these operating bias levels, magnetic bubbles are moved about within the material by application of an in-plane rotating magnetic field. Patterns of permalloy elements laid down on the surface of the material provide paths along which the magnetic bubbles move in response to the rotating field.
Proper use of the bubble memory devices depends in part on the ability to completely control the generation of the bubbles within the material. Methods have been developed for controlled generation of bubbles at operating bias field values. In a book entitled, "Magnetic Bubbles" by A. H. Bobeck and E. Della Torre, a bubble generator is described at pages 177-184 which uses a special full-disk permalloy element in combination with a series of bar and T bar elements. As an example, a seed bubble permanently trapped and associated with the full disk element rotates with the positive polarity portion of the magnetization induced in the element by the rotating in-plane field. At some point during rotation of the in-plane field adjacent positive polarities occur in a portion of the full disk element and in a tip of an adjacent but spaced apart bar element from the T bar propagating channel. The positive polarities are close enough to stretch the seed bubble from the full disk element across to the adjacent bar element. Further rotation of the field causes a negative polarity to rotate between the rotating positive polarity on the full disk element and the positive polarity in the T bar propagating channel until the stretched bubble breaks leaving a seed bubble with the full disk element and a newly formed bubble in the T bar channel. With this method, a seed bubble must be maintained and associated with the full disk element at all times contributing to the unreliability of the device in the event that the seed bubble is destroyed or otherwise lost. Further, a new bubble is generated with every rotation of the in-plane field whether one is required or not.
Another means of generating bubbles requires the use of current carrying conductor loops such as those described in the above mentioned reference by Bobeck and Della Torre. When current is pulsed through the loop, the net bias field of the material is lowered locally, (directly beneath the current loop), thereby forming a bubble in the material. However, fabrication of separate layers of current carrying conductors on the surface of the bubble material and associated control circuitry increases manufacturing and design complexity driving up the cost and unreliability of bubble memory devices.